Reports startup script exit status to a GCE guest attribute so the controller waits for completion before launching the agent. The snippet receives the exit code as $args[0] and must PUT it to:
http://metadata.google.internal/computeMetadata/v1/instance/guest-attributes/startup-script/status

Clear this field to disable waiting.

The default uses Invoke-RestMethod, which is built into PowerShell. Any tool capable of an HTTP PUT to the URL above will work. Keep $args[0] as the exit code placeholder.